<P>In aim to determine whether stem/progenitor cells are present in adult thyroid, we first focused on side population (SP) cells. SP cells are characterized by their ability to efflux the vital dye Hoechst 33342 when analyzed by flow cytometry, due to expression of the ATP binding cassette (ABC)-dependent transporter ABCG2, and are highly enriched for stem/progenitor cell activity. SP cells have been identified in various hematopoietic and non-hematopoietic adult tissues, the latter including the liver, skeletal muscle, lung, kidney, and mammary gland.</P> <P>We have identified SP cells in the adult mouse thyroid at a approximately 1% of a total population of cells, which highly express ABCG2 and the stem cell marker genes encoding nucleostemin and Oct4 as examined by RT-PCR and real-time PCR. On the other hand, the expression of genes encoding the thyroid differentiation markers, thyroid peroxidase, thyroglobulin, and TSH receptor, and two transcription factors, T/EBP and PAX8, critical for thyroid specific gene expression, are low in the SP fraction of cells. Further, we demonstrated that thyroid SP cells are composed of two populations of cells; CD45(-)/c-kit(-)/Sca1(+) and CD45(-)/c-kit(-)/Sca1(+), in which CD45 and c-kit are considered to be markers for hematopoietic lineages. Sca1 (stem cell antigen 1) is a marker, expressed on bone marrow, muscle and mammary gland-derived SP cells. Cells expressing ABCG2 reside in the intrafollicular space of the thyroid gland as determined by <I>in situ</I> hybridization. Double immunofluorescence study further demonstrated that approximately a half and a few ABCG2-positive cells are also positive for vimentin and calcitonin, respectively, suggesting that thyroid SP cells may be derived from lineages of mesenchymes and/or C cells. After nine weeks under three-dimensional collagen thyroid primary culture conditions, non-SP cells formed epithelial arrangement and follicle-like structures that are immunoreactive for T/EBP and thyroglobulin, while SP cells demonstrated very few morphological changes. These results demonstrate that thyroid possesses SP cells that may represent stem/progenitor cells. Our current focus it to determine whether SP cells contribute to thyroid tissue regeneration using mice that have undergone partial thyroidectomy in conjunction with flow cytometry, gene expression analysis by RT-PCR and qPCR, and three-dimensional thyroid primary culture studies.</P>